Abstract

The involvement of Ca²⁺ in insulin-mediated glucose uptake is uncertain. We measured Ca²⁺ influx (as Mn²⁺ quenching or Ba²⁺ influx) and 2-deoxyglucose (2-DG) uptake in single muscle fibers isolated from limbs of adult mice; 2-DG uptake was also measured in isolated whole muscles. Exposure to insulin increased the Ca²⁺ influx in single muscle cells. Ca²⁺ influx in the presence of insulin was decreased by 2-aminoethoxydiphenyl borate (2-APB) and increased by the membrane-permeable diacylglycerol analog 1-oleyl-2-acetyl-sn-glycerol (OAG), agents frequently used to block and activate, respectively, nonselective cation channels. Maneuvers that decreased Ca²⁺ influx in the presence of insulin also decreased 2-DG uptake, whereas increased Ca²⁺ influx was associated with increased insulin-mediated glucose uptake in isolated single cells and whole muscles from both normal and insulin-resistant obese ob/ob mice. 2-APB and OAG affected neither basal nor hypoxia- or contraction-mediated 2-DG uptake. 2-APB did not inhibit the insulin-mediated activation of protein kinase B or extracellular signal–related kinase 1/2 in whole muscles. In conclusion, alterations in Ca²⁺ influx specifically modulate insulin-mediated glucose uptake in both normal and insulin-resistant skeletal muscle. Moreover, the present results indicate that Ca²⁺ acts late in the insulin signaling pathway, for instance, in the GLUT4 translocation to the plasma membrane.